In modern communication systems it is endeavored to use electrical components which are as cost-effective as possible and the power loss of which is moreover low. One example of such a component is a radiofrequency power amplifier in mobile radio base stations. Base stations require corresponding output powers for their transmission signals of up to 200 watts. These output powers are provided by radiofrequency power amplifiers for base stations, which are among the most expensive individual components of a base station. In order to operate such amplifiers in a range with high efficiency, it is expedient to use the transistors of the radiofrequency power amplifiers in a nonlinear range of their characteristic curve. The term nonlinear range is understood hereinafter to mean that range of the transfer characteristic curve of an electrical component which has a nonlinear transfer response, that is to say in which the amplitude and the phase of the output signal are not proportional to the amplitude and phase of the input signal.
Operating a component in a nonlinear range of its transfer characteristic curve generates a distortion on account of an intermodulation, that is to say a mutual influencing of individual signal components of the input signal. This causes additional frequencies and hence a spectral broadening of the signal output by the component, which can undesirably influence the adjacent channels. In order to avoid intermodulation, it would be possible for example for a power amplifier of the base station to be dimensioned correspondingly and to be modulated in a linear range of its characteristic curve. However, this variant is not recommendable on account of the large chip area in production and the high costs and also owing to low efficiency in operation.
Another possibility for reducing a distortion of the signal to be transmitted includes predistorting the signal to be output. This efficient and very flexible linearization variant is also referred to as “digital predistortion”. In this case, a predistorted signal rather than the actual undistorted useful signal is fed to the amplifier. Owing to the nonlinear transfer response of the power amplifier, the input signal is distorted again. Given a suitable choice of the predistortion, the distortion caused by the amplifier is compensated for, so that the desired amplified useful signal can be tapped off on the output side. A distortion is achieved by a circuit connected upstream of the input of the radiofrequency power amplifier. This “predistorter” distorts the signal to be transmitted in a suitable manner, thereby correcting the distortion on account of the nonlinear transfer response in the radiofrequency power amplifier.
FIG. 4 shows a block diagram with elements in the transmission signal path of a base station. The data BB to be transmitted are fed to a baseband unit 100 at an input 130. The baseband unit 100 contains a modulator 120, inter alia, which codes the data to be transmitted in accordance with a predetermined type of modulation. The type of modulation is prescribed by a mobile radio standard. The coded data are referred to as the digital baseband signal. They are fed to a predistorter 110, which alters an amplitude and, if appropriate, a phase of the digital baseband signal in order to take account of the nonlinear transfer response of components connected downstream.
The predistorted baseband signal that is output is converted into an analog signal and converted onto a carrier frequency by means of a mixer 200 with the aid of a local oscillator signal LO at the input 210. In this case, the mixer 200 may contain different circuits, e.g. an IQ modulator or a polar modulator. Afterward, the frequency-converted signal TX is amplified to the desired output power in a power amplifier 300 and the amplified signal TX′ is output. The radiofrequency power amplifier 300 is operated in a nonlinear range for the signal amplification. Owing to the predistortion of the signal to be amplified by the predistorter 110, the distortions on account of the nonlinear transfer response are ideally compensated for. An approximately undistorted useful signal TX′ on the carrier frequency with the desired output power is present at the output 310.
In order to determine predistortion coefficients which are required for the predistortion of the useful signal, it is expedient for the transfer response, of the electrical components, for example of the radiofrequency power amplifier 300, to be described as accurately as possible. This description generally represents a complex problem since the transfer response of the radiofrequency power amplifier and hence its characteristic curve, as already mentioned, is nonlinear. In addition, the amplifier often has dynamic effects. The dynamic effects essentially arise as a result of a “memory” of the radiofrequency power amplifier. Thus, the signal present on the input side influences the output signal of the power amplifier. The output signal at one point in time is accordingly dependent on the input signal at this point in time and relative to a preceding temporal profile of the input signal.